1. Field of the Invention
The present invention relates to an apparatus for removing soot and nitrogen oxide (NOx) in the exhaust gas of diesel engines. Particularly, the inventive apparatus is equipped with a plasma/catalyst system comprising a diesel particulate filter, plasma reactor and catalytic reactor for efficiently removing the harmful pollutants in the exhaust gas.
2. Description of the Prior Art
Generally, the diesel engine has an advantage in that it exhausts less pollutant, such as CO and CO2 gases and hydrocarbons, which have been shown to be the cause of global warming. The environment is therefore more protected by the use of the diesel engine, owing to its higher heat efficiency and durability in comparison to the gasoline engine.
However, nitrogen oxide (NOx) in the exhaust gas of the diesel engine causes to generate the photochemical smog, acid rain and ozone (O3). In addition, because the soot particulates in the air are more capable of absorbing light than any other particulates, the soot causes poor visibility and also respiratory disease if such particulates are inhaled in any great amount.
There are various filtering technologies for the exhaust gas of the diesel engine. One method physically filters the soot-particles through a filter. Another method is that NO in the exhaust gas oxidizes to NO2 by using non-thermal plasma, then the soot and NOx are simultaneously eliminated. Further, another method is that hydrocarbon is oxidized or NOx is deoxidized by catalyst. A combination or modification of the various methods is also used.
European Pat. EP0937870 to the Delphi Co. discloses an apparatus for the removal of hydrocarbon, CO and NOx by using a plasma/catalyst system. According to the Delphi""s apparatus, the exhaust gas passes through a primary catalytic layer and plasma reactor and then a secondary catalytic layer sequentially. Through the process, a portion of the hydrocarbon and CO gases is removed by being oxidized in the primary catalytic layer, and NO is converted to NO2 in the plasma reactor. Sequentially, the residual hydrocarbon and CO gases are removed in the secondary catalytic layer by oxidizing with NO2, which is produced in the plasma reactor, and catalyst.
PCT numbers W098/00221 and U.S. Pat. Nos. 5,746,984 and 6,156,162 to Hoard disclose the apparatuses, comprised of a filter and plasma reactor for the treatment of the exhaust from a diesel engine. The particulates collected in the filter are removed by oxidizing with NO2, produced in the plasma reactor, and disposed of in front of the filter. The filter is made of ceramic, zeolite or perovskite coated with copper oxides or barium oxides. The time for reacting the plasma and replacing the filter is determined depending on the increasing the backpressure of the filter (or decreasing the engine output) by a sensor mounted at the outlet of the apparatus as mentioned above. In addition, it discloses the other various plasma reactors.
U.S. Pat. Nos. 5,711,147; 6,038,853 and 6,038,854 to Penetrante et al. disclose a system for removing NOx and particulates by a two-step process. In the first step, NO is oxidized to NO2 during which time the plasma is generated under the presence of O2 and hydrocarbon, and the produced NO2 reacts with soot particles for converting to N2 or CO2. In the second step, the residual NO2 passes through a catalytic layer under the presence of hydrocarbons for deoxidizing to N2. Especially, U.S. Pat. No. 6,038,854 discloses an implementable example of the plasma/filter/catalyst system.
U.S. Pat. Nos. 4,902,487 and 5,943,857 and PCT WO 00/21646 to Johnson Matthey Co. disclose a system for treating the exhaust gas comprised of a catalyst and a diesel particulate filter. The particulate substances are collected through the filter and oxidized by NO2, which is produced in the catalytic layer installed in the front end of the filter at the normal temperature for removal. It is possible to use a plasma reactor instead of the catalyst. In this case, ozone (O3) is also produced to oxidize with soot.
PCT numbers WO 00/43645 and WO 00/43102 to AEA Technology disclose a method for the removal of particulates and NOx by using a plasma reactor and catalyst. When plasma is generated by adding hydrocarbons, NO is transformed to NO2, then the generated NO2 oxidizes the soot to remove. The residual NOx is removed by catalyst. A dielectric barrier is disposed between the electrodes without installing a filter.
PCT number WO 00/29727 to Engelhard Co. discloses a system for removing NOx (excluding the soot) by using a plasma reactor and catalyst. This system produces NO2 by plasma under the highly concentrated hydrocarbon to deoxidize with catalyst. It discloses that when a molar ratio of carbon atom to NOx is 5:1 in the exhaust gases, approximately 50% of NOx is transformed to N2.
Ford Co. also reported that when a molar ratio of NOx to carbon atom is about 1:5 in the exhaust gas (the same ratio as the Engelhard Co.) by using plasma and catalyst, which is developed by itself, the transforming rate of NOx to N2 and hydrocarbon to water and CO2 is 50% and 30%, respectively.
In the International Symposium on High Pressure, Low Temperature Plasma Chemistry held at Greifswald, Germany (Sep. 10xcx9c13, 2000), Institute of Nonthermal Plasma Physics and Th. J. Heimbach GmbH disclosed an apparatus for removing soot as shown in FIG. 1 in which a porous element 11 for collecting the soot is used as an electrode. The other electrode 12a coated with dielectric barrier is inserted into the center of the filter cell to generate plasma. Thus, the filter cell itself is used as a plasma reactor for removing the collected soot on the filter (HAKONE VII).
For the detailed descriptions referring to FIG. 1, the soot in the exhausted gas is filtered through the porous element 11 used as an electrode. The NO in the exhaust gas is transformed to NO2 by the plasma generated between the porous element 11 and electrode 12, coated with a dielectric barrier, present at the center of each filter cell. When the produced NO2 passes through the porous element 11 electrode, the collected soot on the filter is oxidized and thereby removed.
According to the conventional systems for simultaneously removing soot and NOx, NO in the exhaust, gas is transformed to NO2 through the reaction as represented in the following reaction formula 1, and then the soot and NO2 are simultaneously removed by the reaction as represented in the following reaction formula 2. The residual NO2 is reduced to N2 by the catalyst.
Reaction Formula 1:
NO+xc2xd O2xe2x86x92NO2
Reaction Formula 2:
C(soot)+NO2xe2x86x92CO+NO or CO2+xc2xdN2 
Because the reaction for removing the soot and NOx occurs while the exhaust gas containing NO2 is passing through the layer of soot collected on the filter, the reacting time is very short. Therefore, the soot in the exhaust gas is incompletely treated.
In order to overcome the problems as described above, the present invention provides a plasma system for completely removing the soot and NOx in the exhaust gas of a diesel engine.
An objective of the present invention is to provide an apparatus for removing soot and nitrogen oxide in the exhaust gas of diesel engines by a diesel particulate filter adopting a honeycomb type porous element and having at least one pair of electrodes. A plasma reactor for generating a predetermined amount of plasma is mounted downward on the diesel particulate filter. A catalytic reactor is filled with a catalyst selected from 1B metal group and mounted downward on the plasma reactor. A means of supplying hydrocarbon is connected to a proper position upward on the plasma reactor.
The electrodes in the diesel particulate filter are comprised of a rod-type electrode able to be inserted into the center of the passage section, and a pipe-type electrode mounted to the outside of the passage section. The electrodes in the diesel particulate filter could enable a rod-type electrode to be mounted in a parallel direction, directly opposite from the passage section.
The plasma reactor generates plasma by a method selected either one of corona discharge, microwave radiation, UV radiation, radio frequency (RF) discharge, dielectric barrier discharge, glow discharge, surface discharge, or plasma jet. The plasma reactor selectively applies either one of direct current, direct current pulse or alternating current as a power source. The plasma reactor is equipped with an electrode comprising an iron sheet coated with a ceramic layer, which is an insulator. The catalytic reactor is filled with silver catalyst.
A means for supplying hydrocarbon is to supply hydrocarbon from a separate hydrocarbon source or unburned residual fuel during the post injecting process of diesel engine if a separated hydrocarbon source is not provided. The supplied hydrocarbon can be selected either one of propane, propylene or unburned residual diesel fuel. The hydrocarbons have 2-20 carbon atoms.
An apparatus for removing soot and nitrogen oxide in the exhaust gas of diesel engines is comprised of: a plasma reactor for generating plasma from supplied AC power source; a diesel particulate filter connected downward on the plasma reactor, a catalytic reactor for adopting a predetermined catalyst connected downward on the diesel particulate filter, and a means for supplying hydrocarbon connected to a proper position upward on the plasma reactor.